$sql = "SELECT a FROM `table_a`";
首先检测搜索得到的字符串的编码类型
mb_detect_encoding($row['a'], array("ASCII",'UTF-8',"GB2312","GBK",'BIG5'));
如果得到的编码类型不是"UTF-8",则无法使用json_decode进行编码。
当编码为"ASCII",转换成"UTF-8":
iconv('ASCII','UTF-8',$row['a']);
当编码为"GB2312"/"GBK",转换成"UTF-8":
urlencode(iconv('ASCII','UTF-8',$row['a']));
再使用json_encode